Development of information communication technology increasingly requires improved performances of computers and microprocessors used for the technical fields.
The performances of computers and microprocessors are determined by the performance of a central processing unit (CPU). Especially, the performance of an ALU constructing the CPU is very important.
FIG. 1 shows a structure of an ALU generally used. As shown in FIG. 1, the ALU is composed of a combination of semiconductor logic circuits including an exclusive OR gate 11, an AND gate 12, an OR gate 13, a full adder 14 and a multiplexer 15. The logic circuits determine digital output values based on to voltage states.
However, the ALU including the semiconductor logic circuits has a relatively low operating speed. To significantly improve the performance of the ALU, it is more preferable to determine a digital value using a voltage pulse than to determine the digital value according to a voltage state.
There has been proposed a superconductor rapid single flux quantum logic device as a logic circuit that determines a digital value using a voltage pulse. A half adder or a T flip-flop using the superconductor rapid single flux quantum logic device has been also proposed.
FIG. 2 shows a circuit configuration of a conventional half adder using the superconductor rapid single flux quantum logic device as a logic circuit. The half adder is rapidly operated at an operating speed of approximately several tens GHz. In FIG. 2, circuit components denoted by J represent Josephson junctions.
Josephson junction is obtained by weakly combining two superconductors with each other. When an input signal is applied to the Josephson junction, a current of more than a predetermined level is flowed through the Josephson junction to generate a very short voltage pulse of several ps. Accordingly, a digital logic gate having a high operating speed of several tens to hundreds GHz can be constructed using the Josephson junction.
In addition to the aforementioned half adder, a T flip-flop using the superconductor rapid single flux quantum logic device as a logic circuit has been proposed. This T flip-flop is operated very rapidly at an operation speed of 770 GHz.
Recently, there are attempts to utilize the half adder using the superconductor rapid single flux quantum logic device as a logic circuit for ALUs.